The conventional oil pressure door closer is generally composed of a main body fastened with the top rail of a door, and a crank arm fastened at one end thereof with the main body and at other end thereof with a door frame. The main body has two protruded blocks which are made integrally at both ends of the main body and are provided with a fastening hole for fastening the main body with the top rail of the door in conjunction with a screw which is engaged with the fastening hole. The protruded blocks undermine the esthetic effect of the door. There is an improved oil pressure door closer which is provide with the protruded blocks that are concealed by the main body of the door closer. This improved door closer is defective in design in that the main body and the locating structure of the door closer are fastened together by a bolt, thereby making the fastening bolt rather vulnerable to a tangential force brought about by the main body and the locating structure at the time when the door is opened. As a result of the bolt being exerted on by the tangential force for a prolonged period of time, the bolt is prone to become loosened to result in the detachment of the main body of the door closer with the door.